That's where the two position control valve for the Sensenich C-2FM two position prop goes. The oil to the prop is connected where that allen plug is...
Bigger footprint for soft ground but not too large a diameter for the low horsepower. Too large a diameter can put you on the wrong side of the power...
Everyone knows and sets their own limits. Only they know what they are, but when that time comes there will be no question in that person's mind that...
Once you do that and find the amount which satisfies you, do a weight and balance calculation. You will find by placing some weight at the tail, you...